+1 for Renaissance Bangkok Ratchaprasong
You cant go wrong with that one.... But check if they have a promotion running.. not worth to use the awards if they run a promo for ~3000thb...
There is also a new cat 4 Marriott opening this year in Thailand at the east coast. Marriott Rayong... not really much information around yet.
I heard that this may not open until sometime in 2012 - believe that it is going to be owned by the same group that owns the JW in Bangkok. Not exactly sure where it will be located in Rayong, but would not be my first choice for location.